Circuit configuration having a feedback, fully-differential operational amplifier

ABSTRACT

The present invention relates to a circuit configuration having a feedback operational amplifier (AMP), which is implemented as fully differential, for amplifying an input signal differentially input to the circuit configuration and for outputting the amplified input signal as a differential output signal. In order to increase the freedom in setting the input common mode voltage, according to the present invention, a combination made of a coupling resistor (R 1   b ) and a level shifter (I 1   b , Nsfb) connecting the positive amplifier output (y 1 ) to the inverting amplifier input (x 2 ) and a combination made of a coupling resistor (R 1   a ) and a level shifter (I 1   a , Nsfa) connecting the negative amplifier output (y 2 ) to the noninverting amplifier input (x 1 ) are provided.

BACKGROUND OF THE INVENTION

1. Area of the Invention

The present invention relates to a circuit configuration having afeedback operational amplifier, implemented as fully differential, foramplifying an input signal input differentially into the circuitconfiguration and outputting the amplified input signal as adifferential output signal.

The term “operational amplifier” as defined in the present invention isto be understood very broadly as a configuration capable of amplifyingan electrical variable such as a voltage. In particular, it refers toamplifiers, for example, in which a signal applied to the amplifierinput is provided having relatively high voltage amplification at theamplifier output. This open amplification (“open loop again”) may be inthe magnitude of approximately 10⁴ through 10⁵, for example.

An essential characteristic of the circuit configuration according tothe species is that the circuit amplification, i.e., the ratio betweenoutput signal and input signal, is practically completely independent ofthis open amplification (except for transient occurrences) and is solelypredefined by an additional (external) configuration or a “feedbacknetwork” of the operational amplifier. The feedback network is formed inthe circuit configuration according to the species from the totality ofcoupling, feedback, and decoupling paths.

The feedback network determines the resulting circuit amplification. Inthe simplest case, the feedback network comprises a configuration of oneor more (ohmic) resistors. Alternatively or additionally, othercomponents such as capacitors and/or inductors may be provided toproduce the feedback network. Very generally, these components providedto produce the feedback network are thus identified in the following asimpedances.

2. Description of the Prior Art

The fully differential implementation of the operational amplifier,which is advantageous for many applications, means that the differenceof the potentials applied to the two amplifier inputs (amplifier inputvoltage) is provided amplified by the open amplification at the twoamplifier outputs as the amplifier output voltage, no galvanic couplingbetween the amplifier inputs and the amplifier outputs being performedby the operational amplifier itself. In order to provide definedpotential ratios at the output of such an operational amplifier, inpractice, the output common mode voltage is typically set to apredefined value. This output common mode voltage, referred to in shortin the following as the “output CM”, is defined as the mean value of thetwo voltages provided at the amplifier output (each in relation to areference potential, e.g., a supply potential). Fully-differentialoperational amplifiers typically have a CM setting input to be impingedby a setting voltage for setting the output CM to a value whichprecisely corresponds to this setting voltage applied to the settinginput.

If the feedback network provides resistive feedback paths from theamplifier outputs to the amplifier inputs, these feedback paths not onlyinfluence the resulting circuit amplification, but rather also influencethe input-side potential ratios of the operational amplifier through thegalvanic coupling which is thus more or less implemented. In many cases,this is a desired effect (or side effect), for example, to set the inputcommon mode voltage, also referred to in the following in short as the“input CM”, to the same value as the output CM. This is true inparticular if the input CM was not yet defined using the input-sidecircuitry of the operational amplifier alone. In this context, it is tobe noted that for some operational amplifier circuits, both the input CMand also the output CM are to each be kept reliably in a specific rangeto ensure perfect function of the operational amplifier. These rangesare at least within the range predefined by two supply potentials of theoperational amplifier. For the output CM, it is additionally favorablein regard to a large control range of the operational amplifier if it isapproximately in the middle of the range predefined by the two supplypotentials. In this case, it is disadvantageous under certaincircumstances, however, if the output CM at approximately half of thesupply voltage causes an equally large input CM via resistive feedbackpaths.

OUTLINE OF THE INVENTION

It is therefore an object of the present invention to increase thefreedom in setting the input CM in a circuit configuration of the typecited at the beginning.

This object is achieved according to the present invention by acombination made of a coupling resistor and a level shifter whichconnects the positive amplifier output to the inverting amplifier inputas well as a combination made of a coupling resistor and a level shifterwhich connects the negative amplifier output to the noninvertingamplifier input.

The level shifter thus connected between an amplifier output and acoupling resistor has the object of applying the potential existing atthe amplifier output to the coupling resistor shifted by a predeterminedamount.

Through the measures according to the present invention, the input CMmay advantageously be predefined differently from the output CM. In manyapplications, this represents a significant advantage.

The circuit configuration according to the present invention mayparticularly be provided as a functional block of an integrated circuitconfiguration, in particular an integrated circuit configurationmanufactured in CMOS technology.

The influencing of the input-side potential ratios on the basis of theoutput CM achieved by the present invention is particularly veryadvantageous if at least one part of the coupling paths are formed byimpedances having a capacitive component, in particular if the couplingpaths are of a purely capacitive type and the input-side potentialratios, particularly the input CM, would be defined rather poorly or notat all without the measures according to the present invention.

Coupling paths of a purely capacitive type may be formed, for example,by a first capacitor, which connects a first circuit input to thenoninverting amplifier input, and by a second capacitor, which connectsthe second circuit input to the inverting amplifier input.

Furthermore, at least a part of the feedback paths may be formed byimpedances having a capacitive component, in particular, feedback pathswhich comprise a first capacitor and a second capacitor may also beprovided here, the first capacitor connecting the negative amplifieroutput to the noninverting amplifier input and the second capacitorconnecting the positive amplifier output to the inverting amplifierinput.

Further coupling paths or decoupling paths may be connected in parallelto each of the coupling paths and/or decoupling paths explained above.Such further coupling paths may be permanent or alternately may beconnectable and disconnectable to the feedback network via a switchingelement such as a transistor. The alternate connection and disconnectionmay advantageously be used for operational change of the circuitamplification. The combinations according to the present invention, eachof which is formed by a coupling resistor and a level shifter accordingto the present invention, may provide significant advantages inconnection with the connection and disconnection of additional couplingpaths, as is obvious from the exemplary embodiment described below.

In a preferred embodiment, the coupling resistors are each implementedas ohmic resistors having a resistance value of more than 1 MΩ. Using acomparatively large resistance value of this type, the influence of thiscoupling resistor on the feedback or the circuit amplificationdetermined thereby may be kept low. This is particularly advantageous ifthe feedback paths are to be implemented essentially capacitively.

In one embodiment, both the coupling paths and also the feedback pathsare implemented essentially capacitively. For example, both two couplingpaths, which connect a first circuit input to the noninverting amplifierinput and a second circuit input to the inverting amplifier input, andalso two feedback paths, which connect the negative amplifier output tothe noninverting amplifier input and the positive amplifier output tothe inverting amplifier input, may each be formed by a couplingcapacitor, one of the combinations provided according to the presentinvention of a coupling resistor and a level shifter being connected inparallel to each of the two feedback paths.

For a specific operating frequency range (frequency range of the inputsignal to be amplified), the influence of the coupling resistor on thecircuit amplification may be kept negligibly small through suitableselection of the resistance value of the coupling resistor or thecapacitances of the coupling resistors.

Using an amplifier which is essentially capacitively coupled both at itsinput and also in regard to the feedback, in particular signalscontaining signal components having high frequencies may be amplified(while simultaneously suppressing low-frequency components). In oneembodiment, the circuit configuration is used for amplifying inputsignals and having a frequency or having frequency components which aregreater than 1 MHz (e.g., a few tens to 100 MHz). The decoupling pathsof the circuit configuration may be formed in the simplest case bydirect line connections between the amplifier output and the circuitoutput, which connect the negative amplifier output to a first circuitoutput and, in addition, the positive amplifier output to a secondcircuit output.

The operational amplifier may be configured as an inverting amplifierwhich is essentially capacitively coupled overall. The present inventionparticularly provides special advantages if the operational amplifierhas a differential input stage of a type known per se (inputdifferential pair, preferably PMOS), since in this case an output CMwhich corresponds to half of the supply voltage is advantageous, but anequally large input CM would often be unfavorable. This is particularlytrue for circuit configurations or manufacturing technologies forintegrated circuits in which very low supply voltages are provided(e.g., 1.5 V or less).

In a preferred embodiment, the level shifters each comprise acurrent-carrying resistor element or a, current-carrying transistor.

The current flow is preferably provided using a constant current source.The resistor element may be formed by a powered channel of afield-effect transistor (FET), for example, particularly by an FETchannel powered using a constant current source. Such an FET (e.g.,MOSFET) may, for example, be incorporated as a source sequencer in thecircuit configuration in such a way that its gate is connected to theaffected amplifier output, whose source is connected to the couplingresistor and, in addition, to a current source (e.g., constant currentsource) and whose drain is connected to a predefined reference potential(e.g., a supply potential of the circuit configuration or theoperational amplifier).

The two level shifters are preferably constructed identically, so thatidentical potential shift absolute values are ensured for the twofeedback paths in a simple way.

In one embodiment, the operational amplifier has a setting input forsetting the output CM.

According to a first variation, in regard to the setting of the outputCM, the operational amplifier is connected to a first supply potentialand the second supply potential for its supply and the output CM is setas a predetermined fraction of the supply voltage. It is at leastfavorable for this purpose if the fraction is in the range from 40% to60%, particularly at least approximately 50%.

According to another variation, the operational amplifier is connectedto a first supply potential and a second supply potential for its supplyand the output CM is set as the supply voltage reduced by apredetermined reduction voltage. It is at least favorable for thispurpose if the reduction voltage is in the range from 40% to 60% of arated supply voltage of the operational amplifier, particularly at least50% of the rated supply voltage.

In one embodiment, the coupling paths comprise impedances connectableand disconnectable via one transistor each for changing a circuitamplification. In this case, a special advantage of the presentinvention may result because through the level shifting of the input CMin relation to the output CM, a potential exists at a transistorterminal connected to the affected amplifier input, which allows turningon (transistor conducts) and turning off (transistor blocks) withoutproblems. If the transistor is an FET, for example, whose source (ordrain) is connected to the affected amplifier input and which isswitched as conductive and nonconductive through a variation of the gatepotential, an unfavorable source potential (or drain potential) mayresult in this FET itself not being able to be brought into thecompletely conducting and/or blocking state when the gate potential isvaried over the entire supply voltage range. This set of problems isparticularly significant for comparatively small supply voltages of theaffected circuit configuration. The present invention may provide aremedy here in that the input CM and thus finally the two input-sidepotentials at the operational amplifier are shifted in a desireddirection.

BRIEF DESCRIPTION OF THE DRAWING

The present invention is described further in the following on the basisof exemplary embodiments with reference to the attached drawing.

FIG. 1 is a circuit diagram to illustrate a capacitively coupledinverting amplifier according to a typical embodiment, and

FIG. 2 is a circuit diagram of an amplifier according to the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a circuit configuration implemented as part of amicroelectronic integrated circuit for amplifying a high-frequency inputsignal and for outputting the amplified input signal as an outputsignal.

The input signal is input as a differential voltage signal at a firstcircuit input inp and a second circuit input inn and coupled via a firstcoupling path comprising a capacitor C1 a and a second coupling pathcomprising a capacitor C1 b to a noninverting amplifier input or aninverting amplifier input, respectively, of an operational amplifierAMP.

The operational amplifier AMP is implemented as fully differential andaccordingly has two amplifier outputs, namely a negative amplifieroutput, which is connected directly to a first circuit output outn, anda positive amplifier output, which is connected directly to a secondcircuit output outp.

A feedback path comprising a capacitor C2 a is connected between acircuit node y2, which is connected directly to the negative amplifieroutput, and a circuit node x1, which is connected directly to thenoninverting amplifier input. A feedback path comprising a capacitor C2b is connected between a circuit node y1, which is connected directly tothe positive amplifier output, and a circuit node x2, which is connecteddirectly to the inverting amplifier input.

The capacitors C1 a and C1 b, which are dimensioned at identicalcapacitance, and the capacitors C2 a and C2 b, which are alsodimensioned at identical capacitance, form a symmetrical externalcircuit (feedback network) of the operational amplifier AMP, whichdefines the resulting circuit amplification, i.e., the ratio of theoutput voltage at the circuit output outn, outp to the input voltage atthe circuit input inp, inn independently of the open amplification ofthe amplifier AMP. An input differential pair of the amplifier AMP ispreferably implemented in PMOS.

In the exemplary embodiment shown, the circuit amplification may bechanged by simultaneously connecting and disconnecting to furthercoupling paths, each of which comprises a series circuit made of acapacitor and a field effect transistor. The coupling path running fromthe input inp to the circuit node x1 may be connected via a transistorNsa and a capacitor C3 a, and the coupling path running from the inputinn to the circuit node x2 may be connected via a transistor Nsb and acapacitor C3 b. The reference sign Vs used in the figure symbolizes theswitching voltage supplied in this case for this connection anddisconnection to the gate terminals of the transistors Nsa and Nsb, inrelation to a first supply potential GND (negative here) of the circuitconfiguration. For the sake of simplicity of the illustration, thesupply of the operational amplifier AMP with this first supply potentialGND and a second supply potential vdd is not shown in the figure.

A setting voltage Vcm for setting the output common mode voltage (outputCM) of the operational amplifier AMP is also provided in a circuit partin relation to the first supply potential GND and supplied to theoperational amplifier AMP at a setting input provided for this purpose.In a way known per se, the output CM of the operational amplifier AMP isthus set to the value Vcm.

The output CM is expediently set to the mean value of the two supplypotentials GND and vdd to achieve the greatest possible control range(variation of the output voltage).

The input CM, which at the typically very large open amplification ofthe operational amplifier AMP practically corresponds to the (nearlyidentical) potentials at the circuit node x1 and x2, is also to be fixedfor perfect functioning of the amplifier AMP. This is achieved in thecircuit configuration shown by resistors R1 a and R1 b, which are eachconnected in parallel to the feedback paths C2 a and C2 b. Theseresistors couple the amplifier output (y1, y2) galvanically to theamplifier input (x1, x2), so that the input CM is equal to the outputCM. These resistors are selected having a comparatively high resistancevalue (e.g., a few MΩ), so that their influence on the feedbackcharacteristic is practically negligible.

The resistors R1 a and R1 b are used solely for setting the input CM tothe value of the output CM. This type of CM setting has two advantagesin the circuit configuration shown, however.

Firstly, the input CM, which is at 50% vdd, is unfavorable for thedifferential input stage typically provided in an operational amplifier.

In addition, a gate source voltage for the MOSFET switches Nsa, Nsbsufficiently large to bring these transistors into the perfectlyconducting state may not be achieved under certain circumstances. Thisset of problems particularly results for comparatively small supplyvoltages (=vdd−GND) and may result in these switching transistors havinga larger or smaller intrinsic resistance (in the conductive state). Inaddition, this undesired resistance of the source-drain route of theswitching transistors Nsa, Nsb varies with a variation of vdd possiblyoccurring in operation of the circuit configuration and with amanufacturing variation (tolerance) of the usage voltage of thetransistors Nsa, Nsb. The transistors Nsa and Nsb thus have an undesiredinfluence, which is additionally difficult to foresee, on the couplingcharacteristic and thus the resulting circuit amplification.

The problems described above of the circuit configuration from FIG. 1may be corrected by a modification described in the following withreference to FIG. 2.

In the following description of an exemplary embodiment of the presentinvention, identical reference signs are used for identically actingcomponents. Essentially only the differences to the embodiment describedabove are discussed and otherwise reference is expressly hereby made tothe preceding description.

FIG. 2 shows a circuit configuration whose function essentiallycorresponds to the function of the amplifier configuration describedwith reference to FIG. 1. In particular, the input signal guided viacoupling paths to the input of an amplifier AMP is again provided as anoutput signal, amplified by a circuit amplification, the circuitamplification being defined practically exclusively by the capacitancesof capacitors C1 a, C1 b, C2 a, C2 b (and possibly C3 a, C3 b).

The modification from the embodiment shown in FIG. 1 is that acombination made of a coupling resistor R1 b and a transistor Nsfbacting as a level shifter, which connects the positive amplifier outputor the circuit node y1 to the inverting amplifier input or circuit nodex2, is situated and a combination made of a coupling resistor R1 a and atransistor Nsfa acting as a level shifter is situated between thenegative amplifier output or the node y2 and the noninverting amplifierinput x1.

The drain terminals of these level shifter transistors Nsfa, Nsfb areconnected in the exemplary embodiment shown to the second supplypotential vdd, the gate terminals of these transistors are eachconnected to one of the circuit nodes y1, y2, and the source terminalsare connected to one of two constant current sources I1 a, I1 b inaddition to the particular coupling resistor. The constant currentsources I1 a, I1 b apply a constant current flow via the channel of thetransistors to the transistors Nsfa, Nsfb. As a function of the concreteproperties of the transistors Nsfa, Nsfb, the applied currents result incorresponding gate-source voltages at these transistors, which in turncause a corresponding level shift of the input CM in relation to theoutput CM.

Possibilities for implementing the current sources I1 a, I1 b arewell-known to those skilled in the art and therefore do not require anymore detailed explanation here. In the simplest case, a current sourcemay be formed by an FET operated in saturation, for example.

The capacitance inevitably existing between source and gate of thetransistors Nsfa, Nsfb is significantly smaller (e.g. by a factorgreater than 100 or even greater than 1000) than the actual feedbackcapacitance (C2 a or C2 b) and thus may be neglected in regard to thefeedback characteristic.

In the exemplary embodiment shown, the input CM is somewhat closer tothe first supply potential GND than the output CM. In the circuitconfiguration shown, two advantages are thus achieved.

Firstly, potentials which are especially favorable for the perfectoperation of the operational amplifier AMP exist at the input-sidecircuit nodes x1, x2.

In addition, the potentials existing at the source terminals of theswitching transistors Nsa, Nsb (in relation to GND) are so low thatthese transistors may be switched on without problems, i.e., have a verylow resistance in the switched-on state.

The switching transistors Nsa, Nsb may advantageously be implemented,for example, as MOS switches having thick-oxide variants, which requirecomparatively high voltages for supplying the MOS gates. Without thelevel shifting of the input CM in relation to the output CM provided inthe circuit configuration according to the present invention, separate(larger) supply potentials must be provided for such switchingtransistors or a predefined supply potential must be multiplied, whichwould represent a significant outlay either inside or outside (e.g.,circuit board level) the integrated circuit.

It is noteworthy that any manufacturing tolerances or variations of thegate-source voltage at the level shifter transistors Nsfa, Nsfb and thusthe extent of the level shifting may be largely compensated for by avariation (caused in the same manufacturing process) of thecharacteristic of the switching transistors Nsa, Nsb. Manufacturingtolerances thus do not result in a variation of the intrinsic resistanceof the switching transistors Nsa, Nsb.

In a refinement of the circuit configuration described, the settingvoltage Vcm input to the operational amplifier AMP to set the output CMis not generated as a fraction of the supply voltage but rather as aconstant voltage with reference to vdd (or GND). In other words, in thisrefinement, the voltage Vcm is provided as the supply voltage reduced bya predetermined reduction voltage. The reduction voltage is selected forthis purpose so that Vcm corresponds to half of the supply voltage (asspecified). With this refinement, the influence of a variation of thesupply voltage or the supply potential vdd possibly resulting inoperation of the circuit configuration on the variation of thegate-source voltage of the transistors Nsa, Nsb and thus on thevariation of the intrinsic resistance is drastically reduced.

1. A circuit configuration having a circuit input (inp, inn) for aninput signal to be amplified and a circuit output (outn, outp) foroutputting the amplified input signal as an output signal, the circuitconfiguration having a operational amplifier (AMP) implemented asfully-differential, having a noninverting amplifier input (x1), aninverting amplifier input (x2), a positive amplifier output (y1) and anegative amplifier output (y2), the circuit input (inp, inn) beingconnected via coupling paths to the amplifier inputs (x1, x2) and theamplifier outputs (y1, y2) being connected via decoupling. paths to thecircuit output (outp, outn) and via feedback paths to the amplifierinputs (x2, x1), characterized by a combination made of a couplingresistor (R1 b) and a level shifter (I1 b, Nsfb) connecting the positiveamplifier output (y1) to the inverting amplifier input (x2) and acombination made of a coupling resistor (R1 a) and a level shifter (I1a, Nsfa) connecting the negative amplifier output (y2) to thenoninverting amplifier input (x1).
 2. The circuit configurationaccording to claim 1, wherein at least a part of the coupling paths areformed by impedances having a capacitive component.
 3. The circuitconfiguration according to claim 1, wherein at least a part of thefeedback paths are formed by impedances having a capacitive component.4. The circuit configuration according to claim 1, wherein the couplingresistors (R1 a, R1 b) are each implemented as an ohmic resistor havinga resistance value of more than 1 MΩ.
 5. The circuit configurationaccording to claim 1, wherein the two level shifters (I1 a, Nsfa, I1 b,Nsfb) each comprise a transistor (Nsfa, Nsfb) powered using a constantcurrent source (I1 a, I1 b).
 6. The circuit configuration according toclaim 5, wherein the transistor (Nsfa, Nsfb) is an FET.
 7. The circuitconfiguration according to claim 1, wherein the two level shifters (I1a, Nsfa, I1 b, Nsfb) are constructed identically.
 8. The circuitconfiguration according to claim 1, wherein the operational amplifier(AMP) has a setting input for setting the output common mode voltage(Vcm).
 9. The circuit configuration according to claim 1, wherein theoperational amplifier (AMP) is connected to a first supply potential(GND) and a second supply potential (vdd) for its supply and the outputcommon mode voltage (Vcm) is set as a. predetermined fraction of thesupply voltage (vdd-GND).
 10. The circuit configuration according toclaim 9, wherein the fraction is in the range from 40% to 60%.
 11. Thecircuit configuration according to claim 1, wherein the operationalamplifier (AMP) is connected to a first supply potential (GND) and asecond supply potential (vdd) for its supply and the output common modevoltage (Vcm) is set as the supply voltage (vdd-GND) reduced by apredetermined reduction voltage.
 12. The circuit configuration accordingto claim 11, wherein the reduction voltage is in the range from 40 to60% of a rated supply voltage (vdd-GND) of the operational amplifier(AMP).
 13. The circuit configuration according to claim 1, wherein thecoupling paths for changing a circuit amplification each compriseimpedances (C3 a, C3 b) which are each connectable and disconnectablevia a transistor (Nsa, Nsb).